In a remote control system, signals encoded as infrared rays are used to remotely direct system functions. A remote control transmitter generates control signals, and encodes them as infrared rays which are transmitted to a remote control receiver. The receiver receives the infrared rays and controls a system using the encoded signal information.
FIG. 1 is a circuit diagram of a conventional remote control preamp circuit. The conventional preamp circuit 10 includes a photodiode 11, a differential amplifier 21, resistors 31, 32 and 33 and a capacitor 41.
An anode of the photodiode 11 is grounded, and a cathode thereof is connected to a non-inverting (+) input of the differential amplifier 21. The photodiode 11 receives an infrared ray 12 from an external source and converts the input infrared ray 12 into current which is supplied to the non-inverting (+) input of the differential amplifier 21.
Resistor 31 is likewise connected to the non-inverting (+) input of the differential amplifier 21. A reference voltage (Vref) is applied to the non-inverting (+) input via resistor 31, together with the output of the photodiode 11.
Resistor 32 provides a feedback path between the output Vo terminal and inverting input (-) terminal of the differential amplifier 21. Resistor 32 is connected directly between the terminals, and resistor 33 and capacitor 41 are coupled in series between the inverting input (-) and ground GND.
External noise may enter the non-inverting (+) and inverting (-) inputs of the differential amplifier 21. The difference between the voltages of the noise applied to the non-inverting (+) and inverting (-) inputs is amplified by the differential amplifier 21, and the amplified noise is included in the output signal Vo. Thus, even where the input noise is weak, powerful noise is generated as it is amplified by the differential amplifier 21, which can cause a system, for example a remote control system, to malfunction.